$str = 'Nothing but perl can parse Perl';

print($str =~ /perl/gi ?1:0,"\n");     # Prints '1'
print($str =~ /noth/gi ?1:0,"\n");     # Prints '0'

print($str =~ /perl/i ?1:0,"\n");      # Prints '1'
print($str =~ /noth/i ?1:0,"\n");      # Prints '1'

##</code><code>##

$str = 'Nothing but perl can parse Perl';

while (/perl/gi) {
   # Loops twice
}

while (/perl/i) {
   # Loops forever
}

##</code><code>##

my @matches = ($g ? /$re/g : /$re/);

##</code><code>##

for (...) {
   my ($re, $g) = @$_;
   my @matches = ($g ? /$re/g : /$re/);
   ...
}

##</code><code>##

>perl -le"print qr/.../s"
(?s-xim:...)